Experiments have shown that extreme ultraviolet (“EUV”) light sources, e.g., produced by a laser produced plasma (“LPP”), using a metal such as tin for a plasma source material suffers from erosion of the primary collector mirror. This erosion is due to, e.g., sputtering by energetic source material ions, e.g., tin ions and neutrals created by the LPP. Applicants propose apparatus and methods for addressing this issue.